This invention relates to saw straightening and tensioning and, more specifically, to a novel machine and method for automatically correcting surface defects in a circular saw blade and providing the proper tension distribution to permit the blade to run straight and true at cutting speeds.
Toward the beginning of the 19th century, the circular saw was starting to come into use, revolutionizing wood cutting methods. The speed with which the "buzz saw" was able to cut lumber was totally unheard of and unimaginable, and this new tool was generally considered miraculous. Yet in the beginning the circular saw was far inferior to modern saws in its ability to cut straight and true.
Although manufactured to the best possible tolerances, it was found that when a circular saw was operated at the high rate of rotation necessary for cutting wood, the outer edge of the saw blade had a tendency to deviate from the cutting line either to one side or the other, or to both sides, producing a somewhat wavy cut. There was also a tendency for the edge to start the cut off to one side or the other of the intended line, in which case the blade would tend to lead the cut in that direction. Yet when the saw blade was stopped, it was found to be as straight and true as before.
Eventually it was learned that the invisible defects which resulted in uneven cutting included both minute surface deformities, and improper distribution of residual tension stress in the saw. Substantial tension stress is induced in the blade by centrifugal force when it is rotated at the high angular velocities necessary for cutting wood and other materials. To this tension is added or subtracted the residual stresses in the blade, so that non-uniformity of tension distribution produces uneven strain. Irregularities in the blade surface also interfere with true running as the tension changes when cutting speed is approached. Thus the blade may be deflected from running true by either of these types of defects.
Areas on the surfaces of the blade at which residual tension stresses significantly vary from the normal level of tension in that portion of the blade are commonly referred to as "tight" or "loose" spots. Common geometric deformities include bulges, ridges, kinks and twists.
For many years, the problem of poor tension distribution and invisible surface defects was unrecognized and unsolved. The quality of a circular saw blade and its ability to cut straight and true was largely a matter of chance.
Ultimately it was learned that the operation of a circular saw could be improved by performing certain hammering operations on the saw blade. But it was found that no particular hammering method would improve the operation of all blades, and that varying types of hammering procedures must be utilized to correct different defects. The proper hammering of saws soon became recognized as a highly-skilled craft, an art requiring intuitive analysis of problems and the development of often-unique solutions. There developed a highly-skilled craftsman, the sawsmith, who has since that time been one of the highest paid representatives of all the shop trades. Because of the great skill required, the number of sawsmiths practicing this trade has remained greatly limited and the craft has often been practiced in secret, increasing the aura of mysticism surrounding this function. The sawsmith utilizes a collection of carefully-selected hammers, straight edges, and other tools, as well as senses trained by long experience, to inspect a saw blade for imperfections, mark them as to type and location, and employ the proper hammering patterns and tools to remove these imperfections from the saw. Proper hammering over both surfaces of the saw blade produces a more uniform tension distribution and removes surface flaws, enabling the saw to run straight and true at cutting speed.
While all circular saws are benefited by such hammering, omission of the procedure leaves more pronounced running defects in large circular saws than in small ones, in proportion to mass and diameter. Coupled with the high rate of pay and shortage of sawsmiths, this has dissuaded manufacturers from hammering small circular saw blades, such as those used by the home handyman. If hammered at all, they are only given cursory treatment in an effort to produce a predicted tensioning. Thus, economic pressures have resulted in the marketing of inferior circular saw blades. But the cost of hammering by a sawsmith might in some cases exceed the cost of the saw itself.
In the nearly 200 years of history of the circular saw, many improvements have been made in blades and in methods for their manufacture and treatment. But after the development of the sawsmiths' craft, no further substantial advancement was made in this most difficult and expensive aspect of the treatment of circular saw blades.
The exercise of the sawsmiths' art involves "dishing" the blade, that is, elastically warping or bending it around an axis parallel to its diameter, so that its surface takes the form of a curved cylindrical segment. The sawsmith then applies a straight edge to the blade surfaces at right angles to their curvature, which reveals to a practiced eye the nature and location of defects in the saw. Each defect is marked in a special way which points out, upon later examination, just what type each defect is, so that one may select the tools and techniques best suited for its correction. The sawsmith then places the saw on an anvil and commences hammering the marked areas, guided only by his marks, and he must be extremely careful to hammer in the proper manner and in the correct location. While this rectifying operation is being performed, the sawsmith may also hammer generally over the whole surface of the saw to obtain an overall distribution of tension appropriate to that type of blade.
When the sawsmith applies his straight edge, he is actually finding bulges and depressions representing either physical distortions, or "tight" and "loose" spots on the surface of the saw. The latter are areas of substantially greater or lesser residual tension stress than exists in the surrounding surface. When the saw is dished, any tight spots on the blade surface tend to bulge inwardly from the concave surface, being drawn out as a chord spanning a portion of this surface, while loose spots tend to bulge outwardly. When the blade is laid flat, these areas generally cannot be detected. Kinks, ridges, and other physical distortions inadvertently produced on the saw blade can also be detected by the way they stand up under the straight edge. Additionally, the manner in which the saw curves when dished is, to the skilled hand and eye of the sawsmith, a measure of overall tension distribution in the saw.